An optical signal to noise ratio (OSNR) is a quantity directly related to system performance, no matter in a traditional direct-detection optical communication system or in a coherent optical communication system, so attention has been paid to researches on optical signal to noise ratio monitoring technologies.
A conventional measurement method defined based on an OSNR relies on such conditions that a noise power spectrum is flat and there exists a band containing only noise and containing no signal in the spectrum. With an increase of an optical communication capacity, a transmission length and transmission rate of a coherent optical communication system have been greatly improved than before. More optical nodes will cause larger fluctuation of spectral shapes of noises, and the assumption that the noises are uniformly distributed in the spectrum will face greater challenges. And at the same time, since channel spacings are greatly reduced, it is an unrealistic subject to find a frequency band of which signals may be ignored to measure the noise power. Therefore, measurement of the OSNR in the coherent optical communication system has become a new hotspot of researches.
In an optical communication system, in addition to noises inherent in transmission links, noises introduced due to a nonlinear effect are further included; wherein, the noises include noises introduced by an intra-channel nonlinear effect and noises introduced by an inter-channel nonlinear effect. The noises introduced by the inter-channel nonlinear effect are main factors limiting the accuracy of OSNR monitoring in comparison with the noises introduced by the intra-channel nonlinear effect. The inter-channel nonlinear effect is also referred to as cross-phase modulation (XPM), and nonlinear noises caused by the XPM may further be divided into two types, phase noises and polarization crosstalk.
In OSNR monitoring, the noises introduced by the inter-channel nonlinear effect will cause a deviation between an estimated value and an actual value of the OSNR. For example, the estimated value is lower than the actual value. In order to alleviate influence of the noises introduced by the nonlinear effect on the estimated value of the OSNR, a first method is to nonlinearly compensate for a received signal at a receiver end, and a second method is to calculate white noise power according to pilot signals of different polarization states in the received signal, so as to monitor the OSNR, such a method being based on the premise that the white noise power in different polarization states is identical and the OSNRs are identical.
It should be noted that the above description of the background is merely provided for clear and complete explanation of this disclosure and for easy understanding by those skilled in the art. And it should not be understood that the above technical solution is known to those skilled in the art as it is described in the background of this disclosure.